This application claims benefit of priority under 35USC xc2xa7119 to Japanese patent application No.2000-237163, filed on Aug. 04, 2000, the contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates generally to a charged particle beam exposure system, such as an ion or electron beam exposure system which is used in a process for fabricating semiconductors such as LSIs or VLSIs. More specifically, the invention relates to a low-accelerating-voltage charged particle beam exposure system.
2. Description of the Prior Art
Charged particle beam exposure systems have the function of being capable of forming a high resolution pattern since it is possible to write at a resolving power of a wavelength level of electrons (or ions) which is shorter than light wavelength. On the other hand, since a complete pattern is directly written with small divided pattern beams unlike a mask writing system based on light exposure, there is a problem in that charged particle beam exposure systems take a lot of time to write. However, in view of characteristics that accurate fine line patterns can be formed, the charged particle beam exposure technique has been developed as the next technique to the lithography technique of the light exposure system, or as an important tool to the fabrication of semiconductors in a multi-product small-lot production such as ASIC.
A method for direct-writing a pattern with electron beams mainly uses two systems. That is, there is a system for writing a pattern by scanning the whole surface of a wafer while on-off-controlling small round beams, and a VSB writing system for writing a pattern with electron beams passing through a stencil aperture. As the electron beam writing technique developed from the VSB writing, there has been developed a bulk writing system for preparing a stencil on which repeated patterns are formed as one block and for selecting one of the patterns of the stencil to enable a high-speed writing.
First, as a conventional charged particle beam exposure system, a typical example of an electron beam lithography system of a VSB writing system is shown in FIG. 10 (H. Sunaoshi et al.; Jpn. J. Appl. Phys. Vol. 34 (1995), pp. 6679-6683, Part 1, No. 128, December 1995). Furthermore, in the following drawings, the same reference numbers are given to the same portions to suitably omit the descriptions thereof.
Electron beams 7 emitted and accelerated from an electron gun 11 are arranged as uniform electron beams by means of an illumination lens 15 and pass through a first forming aperture 85 to be formed as rectangular electron beams, and thereafter, projected on a second shaping aperture 89 of a rhombic or rectangular shape by means of a projection lens 87. At this time, the beam irradiation position on the second shaping aperture 89 is controlled by a shaping deflector 21 so that the shape and the area of the second shaping aperture 89 is irradiated with the pattern beams in accordance with CAD data. The beams passing through the second shaping aperture 89 are reduced and projected by means of a reducing lens 64 and an objective lens 66, and a position of the beams on a region of a wafer 14 to be written is controlled by means of a main deflector 95 and a sub deflector 93. In this case, the main deflector 95 controls the interior of a stripe of an irradiation region to be written (main field) with respect to the wafer 14 referring to the position of an XY stage (not shown), and the sub deflector 93 controls the position of a range to be written which is obtained by finely dividing the interior of the stripe (sub-field). Below the objective lens 66, there is an electron detector 33 for detecting secondary electrons and back-scattered electrons (which will be hereinafter referred to as secondary electrons and so forth) which are produced when the wafer 14 is irradiated with the electron beams 7. By processing the detected signals acquired by the electron detector 33, various control parts (not shown) detect an image of SEM, and controls such as adjustment of the trajectories of the beams based thereon are carried out.
Since the electron optical system of an electron beam lithography system 120 shown in FIG. 10 comprises electromagnetic lenses and electrostatic deflectors, it is required to design the electron optical system while sufficiently taking account of the influence of the total optical characteristics of the lenses, the deflectors, the precision of mechanical assembly and contamination. In addition, in order to improve the resolution of beams, there has been widely adopted a system for driving highly accelerated electron beams 7 into a resist on the wafer 14. For that reason, there is caused the proximity effect which is a phenomenon that the incident electron beams 7 reflect on various multilayer thin films deposited on the bottom face of the resist of the wafer 14 to travel above the resist again. This proximity effects causes blurring and deterioration of resolution on the written pattern. Therefore, in the design of the electron beam lithography system, it is essential that the control for correcting the proximity effect be carried out, so that it is required to provide a large-scale system in a control part in addition to the electron optical system. Thus, there is a problem in that the system is complicated and troubles are induced, so that precision is lowered. Moreover, since highly accelerated electrons are used, there is the possibility that the surface of the wafer may be damaged.
In order to eliminate the above described problems in the VSB system of high-accelerating-voltage charged particle beams, an electron beam lithography system of an aperture system using low-accelerating-voltage electron beams has been proposed (Japanese Patent Application No. 10-363071, J. Vac. Sci. Technol. B14 (6), 1996, 3802). The electron beam lithography system proposed in Japanese Patent Application No. 10-363071 is shown in FIG. 11. A first aperture 13 having a rectangular or circular opening is irradiated with electron beams 67 which are emitted and accelerated from an electron gun 11. The electron beams 67 passing through the first aperture 13 travel toward a second shaping aperture 19 comprising the arrangement of a plurality of bulk exposure cell apertures. The beam diameter of the electron beams 67 is adjusted by means of illumination lenses 15a and 15b to such a size which is sufficiently larger than that of any one of cell apertures and in which the electron beams 67 do not interfere with adjacent cell patterns. The illumination lenses 15a and 15b comprise two electrostatic lenses (Einzel lenses), and a negative voltage is applied to the central electrode to use the illumination lenses 15a and 15b. The beams passing through the second illumination lens 15b are controlled to be deflected toward a target position by means of a first shaping deflector 17 so that a target cell aperture of the plurality of cell apertures formed in the second shaping aperture can be selected. The electron beams 67 passing through the second shaping aperture 19 start as cell pattern beams leaving the second shaping aperture 19, and pass through a reducing lens 64 in a state that the beams are returned to an optical axis by a second shaping deflector 21. Above the reducing lens 64, a third shaping aperture 62 is provided for cutting undesired beams scattered by the second shaping aperture 19 and so forth. The electron beams reduced by the reducing lens 64 pass through a pre sub deflector 93xe2x80x2, a pre main deflector 95xe2x80x2, a sub deflector 93, a main deflector 95 and an objective lens 66 to be reduced and projected on the top face of the wafer 14 which is mounted on an XY stage (not shown). The position irradiated with the beams with respect to the position of a pattern to be written on the wafer is controlled by means of the main deflector 95 and the sub deflector 93. In addition, the control voltage of the pre main deflector 95xe2x80x2 with respect to the main deflector 95 is controlled in an addition direction, and the control voltage of the pre sub deflector 93xe2x80x2 is controlled in a subtraction direction, so that total aberration is minimized. The trajectories of the beams downstream of the second shaping aperture 19 are shown in FIG. 12.
Since the electron optical system of the electron beam lithography system 110 shown in FIG. 11 uses the Einzel lenses in its reducing projecting optical system, the electron beams 67 pass through trajectories which are rotation-symmetric with respect to the optical axis as shown in FIG. 12. The pre main deflector 95xe2x80x2, the main deflector 95, the pre sub deflector 93xe2x80x2 and the sub deflector 93 are then associated with each other for deflecting all of the trajectories of the electron beams 67 at the same deflection sensitivity and for causing the produced deflection aberration to be rotation-symmetric with respect to the optical axis. Therefore, the electron beam lithography system 110 is characterized in that it is possible to optimize deflection aberration characteristics in an arbitrary position of trajectories of electron beams to determine the positions of the main and sub deflectors.
However, in the reducing projecting optical system of the electron beam lithography system 110, crossovers 98 and 99 with a high current density are formed downstream of the second shaping aperture 19 as shown in FIG. 12. In addition, this projecting optical system adopts the rotation-symmetry type electrostatic lenses (Einzel lenses) 93 and 95 in a deceleration type focusing mode, the electron beams decelerate in the lenses. These two points cause the beams to blur in the electron beam lithography system 110 shown in FIG. 11 due to chromatic aberration and space-charge effect (particularly, Boersch effect) and the cell aperture image on the wafer to blur, so that there is a problem in that writing characteristics deteriorate.
In order to eliminate the above described problems in the electron beam lithography system of the aperture system using low-accelerating-voltage electron beams, a charged particle beam lithography system having a reducing projecting optical system with a multiple multi-pole lens has been proposed (Japanese Patent laid open No. 2001-093825). An embodiment of the charged particle beam lithography system proposed in Japanese Patent laid open No. 2001-093825 is shown in FIG. 13. In comparison with the electron beam lithography system 100 shown in FIG. 11, the electron beam lithography system 100 shown in FIG. 13 is characterized in that the reducing projecting optical system downstream of the second shaping aperture 19 in the electron optical system is designed with an electrostatic quadrupole lens. A pre main deflector 25a is provided between Q2 and Q3 of an electrostatic quadrupole lens 73.
In the electron beam lithography system 100, the operation after electrons are emitted and are accelerated at an electron gun 11 to be electron beams 68 and until the electron beams 68 pass through an illumination optical system is substantially the same as that of the electron beams 67 of the electron beam lithography system 110 shown in FIG. 11.
After the electron beams pass through the second shaping aperture 19, the interior of the electrostatic quadrupole lens 73 of the reducing projecting lens is irradiated with the electron beams. The quadrupole lens 73 comprises fourth cylindrical electrodes which are provided at angular intervals of 90 degrees. By the action of the quadrupole lens 73, the electron beams pass through different trajectories in X and Y directions to be condensed on a wafer 14. The trajectories of the electron beams between the second shaping aperture 19 and the wafer 14 at that time are shown in FIG. 14. By means of the deflector 25, the incident position in a region to be written (a main field) on the wafer 14 mounted on an XY stage (not shown) is deflected and controlled while referring to the position of the XY stage, and the incident position of range to be written which is obtained by dividing the interior of a stripe (a sub field) is controlled. By adjusting the deflecting voltage ratio of the deflector 25, aberration components produced by deflection are controlled so as to be minimized.
However, if the multi-pole lens is applied to the electrostatic lens of the reducing projecting optical system as the electron beam lithography system 100 shown in FIG. 13 and if electron beams are deflected both in the X and Y directions by means of the same deflector, the electron beams in the X directions and the electron beams in the Y directions pass through asymmetric electron trajectories in a wide-range beam deflection over the wafer by the deflector. Therefore, deflection sensitivity and deflection aberration are greatly asymmetric. In such an optical system, the suppression of the deflection aberration in both of the X and Y directions and the realization of a wide range deflection with high sensitivity impose a great burden on design and fabrication, deteriorate aberration characteristics, and increase the influence of the space-charge effect due to an increase of the optical length.
Moreover, in these optical systems, the electron beams passing through the second shaping aperture 19 form the crossover 98 with a high electron density. Therefore, the Coulomb interaction is conspicuous in this region, so that there is a problem in that the space-charge effect causes the blurring of the cell aperture image to deteriorate writing characteristics.
According to the present invention, there is provided a charged particle beam exposure system comprising: a charged particle beam emitting device which generates charged particle beams with which a substrate is irradiated, the charged particle beam emitting device generating the charged particle beams at an accelerating voltage which is lower than that at which an influence of a proximity effect occurs, the proximity effect being a phenomenon in which a secondary charged particle and/or a reflected charged particle which is/are produced from the surface of the substrate irradiated with the charged particle beams influence(s) an exposure extent of a pattern which is adjacent to a pattern to be written; an illumination optical system which adjusts a beam diameter of the charged particle beams so that density of the charged particle beams is uniform; a character aperture in which an aperture hole is formed in a shape corresponding to a desired pattern to be written; a first deflector which deflects the charged particle beams by an electrostatic field that the charged particle beams have a desired sectional shape and travel towards a desired aperture hole and which returns the charged particle beams passing through the aperture hole to an optical axis thereof; a reducing projecting optical system which forms a multi-pole lens field so that the charged particle beams passing through the character aperture substantially reduce at the same demagnification both in X and Y directions when the optical axis extends in Z directions and form an image on the substrate without forming any crossover between the character aperture and the substrate; and a second deflector which deflects the charged particle beams passing through the character aperture by means of an electrostatic field to scan the substrate with the charged particle beams.